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A week or so back, as a small part of a longer message, Kent wrote-

"Being a professional navigator myself (long time ago) I would say that the
moon should be avoided for finding the MT and longitude due to its
un-regular movements. Only if no other means are available the moon shall be
used, then preferably in or close to the prime meridian."

And I accept that that's a long-held view of many navigators. But is it
justified, I wonder, and why?

Although the Moon's motions are much more complex than those of other bodies
in the sky, they are not "un-regular", in that they are not unpredictable.
Indeed, for nearly two and a half centuries, they have been predictable
enough to use for lunar distances, which is far more demanding of precision
that the ordinary altitude-navigation that's required at this stage in the
observation. But it's true, the navigator has to be more careful, he has to
take account the changes, hour by hour, in dec and HA, using v and d to
interpolate, and allow for the rapid changes in parallax and semidiameter.
So he has more work to do, for the Moon, and has to take a bit more care,
and that alone might be sufficient reason for avoiding the Moon. But that's
all, isn't it? If he is bothered to take that care, observations of the Moon
can be just as precise as observations of any other body. I wonder whether
Kent, from his viewpoint as a professional, would go along with that.

I fully agree that for determining time or longitude, the closer the
observed body is to the prime meridian (due East or West), the better it
will be

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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